1. Field of the Invention
The present invention relates generally to digital modulating methods and a digital modulating apparatus using the same, and particularly to a coding method in digital recording.
2. Description of the Background Art
Coding in digital recording means is utilized to convert a data word sequence with bit intervals of Tb into a codeword sequence with bit intervals of Ts according to a certain rule (a coding rule). Also, coding in which a data word sequence is divided for each information of m bits (m.gtoreq.2) and sequentially converted into a codeword of n bits (n.gtoreq.m) is called block coding. The converted codeword sequence is further converted (modulated) into a recording pattern sequence (recording current form) according to the NRZL rule or the NRZI rule.
Now, coding according to the NRZL rule will be described. In the NRZL rule, a bit "0" corresponds to a certain level (e.g., a low level) and a bit "1" corresponds to another level (e.g., a high level) in a codeword sequence. If a minimum value of the number of continuing same bits in a codeword sequence is expressed as a parameter d (corresponding to a minimum runlength) and a maximum value thereof is expressed as a parameter k (corresponding to a maximum runlength), a minimum length between transitions Tmin and a maximum length between transitions Tmax are expressed as the following expressions, respectively. EQU Tmin=(m/n).multidot.d.multidot.Tb (1) EQU Tmax=(m/n).multidot.k.multidot.Tb (2)
Also, a ratio Tmin/Tb of the minimum length between transitions Tmin and the bit interval Tb of a data word sequence is referred to as a Density Ratio DR, which is expressed as the following expression. EQU DR=(m/n).multidot.d (3)
In digital modulation, it is preferred that the minimum length between transitions Tmin is long and the maximum length between transitions Tmax is short. Also, a larger density ratio DR is more advantageous in high density recording. The relation among a data word sequence, a codeword sequence and a recording pattern sequence is shown in FIG. 15.
Accumulated charge obtained by, assigning charge +1 to a high level of a recording pattern sequence and assigning charge -1 to a low level, sequentially adding charges from the beginning of a recording pattern sequence is called DSV (Digital Sum Variation). That is to say, the DSV shows the difference between the numbers of bits "1" and bits "0" from the beginning to a certain bit in a recording pattern sequence. In the block coding, a total sum of charge in one codeword is called CDS (Code-word Digital Sum). That is to say, the CDS shows the difference between the numbers of bits "1" and bits "0" in one codeword.
Digital modulating methods according to the block coding method include the 8-10 modulating method in which a 8-bit data word is converted into a 10-bit codeword and then NRZ-or NRZI-modulated and recorded, the 8-14 modulating method in which a 8-bit data word is converted into a 14-bit codeword, NRZ or NRZI modulated and recorded, and so forth.
In order to precisely trace recording tracks in reproducing, tracking control is performed such as ATF (Automatic Track Finding), DTF (Dynamic Track Following) and so forth. It is necessary to record a pilot signal in each track of a recording medium for the tracking control. As a method of recording such a pilot signal, there is a recording method in which a pilot signal is superimposed upon digitally modulated data. This method, however, has problems, for example, that a reproduced signal is likely to undergo crosstalk disturbance due to a pilot signal in reproducing.
Accordingly, the 8-10 modulating method, the 12-15 modulating method and so forth are proposed in which control is performed so that DSV varies periodically in digital modulation and the periodical variation of DSV is used as a pilot signal. Such a 8-10 modulating method and a 12-15 modulating method are respectively described in IEEE Transactions on Consumer Electronics, Vol. 34, No. 3, August 1988, pp. 597-605 and Institute of Television Engineers of Japan (ITEJ) Technical Report Vol. 14, No. 41, VIR '90-43, August 1990, pp. 1-6. In these modulating methods, tracking control is enabled with variation periods of DSV being different for each track. According to such modulating methods, a reproduced signal is prevented from undergoing crosstalk disturbance due to a pilot signal.
In the above-described 8-10 modulating method and 12-15 modulating method, the minimum length between transitions Tmin is 0.8 Tb and the density ratio DR is 0.8. Thus, since the density ratio DR is smaller than 1, it is disadvantageous to high density recording.